That's not totally true. They don't just decide to start putting ads up because one day they woke up and thought "holy shit, we have a billion users! how are we going to support this? Quick! Pull out the ads!". It's more like they intend to monetize in the future no matter what but the only thing stopping them is adoption. They need users to adopt the service first before its even monetizeable. I'm sure if ads or whatever model was profitable from day 1 they'd be using it. It's not a matter of purposely deceiving users, it's a matter of at what point do the different models actually yield revenue. Once they get to that point you start seeing the monetization happen. It can be from day 1 or 5 years down the road but all startups need to make money.

Also, they didn't do anything shady here. If you're following you'll know that they both provided ample notice and offered to share the revenue and you can even opt out. I mean, what do we expect here? Are they supposed to have it be opt-in because that's how we like it and we can't be bothered to log in and tick the box to turn it off? We're not paying for anything here and on top of it we're going to tell them how to run the company like we own stock? This is a business, not an open source project or a charity.